Safety device against radiation leakage from microwave apparatus

ABSTRACT

A safety device for microwave ovens is disclosed, which prevents radiation leakage from such an oven, the device being of the kind where a gas lamp is located in the area where radiation leakages may occur, said lamp being connected serially with a potentiometer which cooperates with a controlled diode which drives a relay which cuts the feed to the microwave generating circuit, the safety device being characterized by further comprising a bistable circuit and an amplifier which are connected to one another in cascade arrangement, the bistable circuit and the amplifier being inserted between the potentiometer voltage tap and the control electrode of the controlled diode. By such an arrangement, the sensitivity, the promptness and the reliability of the safety device of the kind described are considerably improved.

United States Patent [191 Mazza 1 SAFETY DEVICE AGAINSTRADIATION LEAKAGEFROM MICROWAVE APPARATUS [301 si nmil sa iqnlir eritriqala Oct. 7. i970.ltaly 30700 A/70 [52] US. Cl 307/117, 325/149, 325/364, 340/228 [51]Int. Cl. H0lh 35/00 [58] Field of Search 307/92, 94, 116, 307/117;219/10, 55; 325/149, 363, 364; 340/228 [56] References Cited UNITEDSTATES PATENTS 10/1967 Herman ..340/228 11/1962 Richardson 340/228 [45.1Aug. 14, 1973 Primary Examiner-Herman J. Hohauser Attorney-Holman 8LStern ABSTRACT A safety device for microwave ovens is disclosed, whichprevents radiation leakage from such an oven, the device being of thekind where a gas lamp is located in the area where radiation leakagesmay occur, said lamp being connected serially with a potentiometer whichcooperates with a controlled diode which drives a relay which cuts thefeed to the microwave generating circuit, the safety device beingcharacterized by further comprising a bistable circuit and an amplifierwhich are connected to one another in cascade arrangement, the bistablecircuit and the amplifier being inserted between the potentiometervoltage tap and the control electrode of the controlled diode. By suchan arrangement, the sensitivity, the promptness and the reliability ofthe safety device of the kind described are considerably improved. I

7 Claims, 2 Drawing Figures MICROWAVE 0* GENERATOR 7 minnow: 14 ms SHEET1 0F 2 Aadmber-fo M41 1 INVENTOR. WW

PATENTEU MIR 1 4 I975 SHEH 2 BF 2 EOCEMZMU In U.S. Patent applicationSer. No. 98,378 filed on Dec. 15, 1970 in the name of the sameapplicant, now Pat. No. 3,679,908, a safety device against radiationleakage from microwave apparatus has been disclosed and claimed, inwhich the operation is based on the ionization by the agency ofmicrowaves of at least a gas lamp which is situated in the neighborhoodof the area where possible radiation leakages from the internal chamberof the apparatus may be experienced. As a matter of fact, this gas lampis electrically and serially connected with a potentiometer from whichthe control voltage is drawn for a controlled diode which, in turn, iselectrically and serially connected with a relay whose energizationcontrols the opening ot the microwave generator feeding circuit. Thus,the variation of the resistance of the lamps, which is due to ionizationby the agency of the microwaves causes the controlled diode to be primedand thus the relay to be energized and the microwave generator to bedeactivated.

An object of the present invention is to provide a device according tothe main patent but having an enhanced sensitivity and improvedpromptness and reliability of actuation, and also to make thesensitivity adjustment of the device less critical, all this having inmind the final objective of improving the safety factor of the device.

Such a desirable improvement of sensitivity, promptness and reliabilityin entering action is obtained, according to this invention, byinserting between the voltage tap of the potentiometer and the controlelectrode of the controlled diode, a bistable circuit and an amplifier,mutually connected in cascade fashion, whereas the sensitivityadjustment is made less critical by providing an impedance match betweenthe circuit connected to the potentiometer and the unblocking impedanceof the controlled diode.

The features of the present invention will be better understood with theaid of the ensuing detailed description of a microwave oven equippedwith a safety device made according to the present invention. Such anoven, which is disclosed and will be described herein by way of exampleonly, is diagrammatically depicted in the accompanying drawings,wherein:

FIG. 1 shows the device in side elevational view, partly in section withthe door set ajar, and

FIG. 2 shows the circuit diagram of the safety antiradiation device withwhich the oven is equipped.

The microwave oven shown in FIG. 1 comprises a substantiallyparallelepipedal body 1, having a cooking chamber 3 in its interior,into which projects the emitting antenna 3 ofa microwave generator 4. Anopening 5, surrounded by four resilient curled laminae 6 and having aclosing door 7, gives access to the cooking chamber. The door 7, hingedand displaceable between a fully open and a fully closed position, has ahandle 14 and a central transparent portion 15 consisting of a glassplate adapted to act as a shield against the microwaves as emitted bythe generator 4 during the food cooking stage. A gas lamp 16, located ina cavity 17 protected by a microwave-pervious wall 18, is an integralpart of the device whose task is to block the operation of the microwavegenerator if radiation leakage occurs. For a more comprehensivedisclosure of the device, reference should be had to the U.S. Patentapplication Ser. No. 98,378, now Pat. No. 3,679,908.

The gas lamp 16 is a part of an anti-radiation safety device whosecircuitry diagram is shown in FIG. 2 and comprises a source ofalternating current (not shown) which, via a pushbutton switch 20, feedstwo circuit branches in parallel, the first of which comprises a diode21 and the parallel connection of a capacitor 22 and a series connectioncomposed by a relay 23 for controlling a normally open contact 24inserted in the microwave generator (4) feeding circuit, a resistor 25,the gas lamp [6, a resistor'26 and a potentiometer 27, whereas thesecond branch comprises an SCR diode 30 and a relay 28 whichrespectively control a normally closed contact 29 inserted in thefeeding circuit of the microwave generator and a normally open contact46 connected in parallel with respect to the SCR diode 30.

The slider of the potentiometer 27 and the control electrode of the SCR,diode 30 are mutually connected by means of a circuit assemblycomprising a snapping circuit of the bistable type and an amplifier. Thebistable circuit comprises two npn transistors 31 and 32 having theiremitters connected to the ground through a common resistors 33 and theircollectors connected to a positive feeding terminal 34 via therespective resistors 35 and 36. The base of the transistor 31 isconnected to the slider of the potentiometer 30 via a variable resistor38, whereas the base of the transistor 32 connected to the ground via aresistor 39 via the parallel connection of a resistor 40 with acapacitor 41. The amplifier comprises, in turn, an npn transistor 42having its base directly connected to the collector of the transistor31, its collector directly connected to the positive terminal 34 and itsemitter connected to ground via a resistor 43 and to the controlelectrode of an SCR diode via a resistor 44. Between the controlelectrode of the SCR diode 30 and the ground there is arranged, inaddition, a reverse bias diode 45.

in operation, until such time as no microwave leakage is experienced,the resistance of the lamp 16 remains high, so that the current flowingtherethrough is low even if it is enough as to keep the relay 23energized (thus the contact 24 is closed) and the transistor 31conducting. By virtue of the conduction of the latter, the transistor 42is cut off and consequently so is the SCR diode 30. The relay 28 is thusde-energized so that the contact 29 is closed and permits that themicrowave generator 4 be fed.

If, however, due to the opening of the door 7 or the malfunction of theblades 6 or breakage of the shielding glass 15 a radiation leakageoccurs, the radiations impinge on the lamp 16 causing the ionizationthereof. The result is a strong resistance drop which causes the flow ofa heavy current composed by a substantially direct current (rectified bythe diode 21 and partially clipped by the capacitor 22) and by animpulsive portion consisting of irregular pulses which are indirectlyoriginated by the radiations. The capacitor 37 blocks the direct currentof the signal and sends to the base of the transistor 31 the alternatingsequence of positive and negative pulses only, as originated by thepulses due to the radiations as aforesaid. Inasmuch as the transistor 31is of the npn type, the first incoming negative pulse switches it to acutoff state (the reasoning should be reserved if the transistor 31would be of the pnp type). The cutoff of the transistor 31, in additionto rendering the transistor 32 conducting, that which has the task ofshortening the switching times of the snap circuit while simultaneouslyrendering it stabler due to the presence of a delay circuity formed bythe resistor 40 and the capacitor 41, causes the conduction of thetransistor 42 and thus the application to the control electrode of theSCR diode 30 of a voltage which is sufficient to prime it. Then therelay 28 is energized and opens the contact 29, the result being thedeactuation of the microwave generator and the closing of the contact 46which keeps the relay 28 energized. In order to reinstate the feed, andthat obviously after having removed the cause of the radiation leakage,it is necessary to press the-button 20 so as to deenergize the relay 28,which, in turn, reopens the movable contact 46 and closes the movablecontact 29 again. By releasing the pushbutton 20 the device is fed oncemore and is brought back to its initial condition.

As desired, the device according to the present invention affords, inaddition to a high sensitivity, an easy adjustment of sensitivity(through the potentiometer) in that, since the input impedance of thesnapping circuit is high, an impedance match is obtained with the sourceof control pulses (drawn between the slider and the lower end of thepotentiometer 27). Likewise, the impedance match is obtained alsobetween the output of the snap circuit and the input of the currentamplifier and also between the output of the latter and the impedance ofthe control circuit of the SCR diode.

What is claimed is:

1. A safety device for sensing and preventing radiation leakage from amicrowave-producing apparatus having a microwave feeding circuit, saiddevice comprising, in combination, a first circuit branch including agas lamp which is sensitive to microwave radiation and which is disposedin a region of possible microwave radiation leakage from themicrowave-producing apparatus, and a potentiometer electrically coupledwith said gas lamp, said potentiometer having a voltage path providing acontrol voltage therefrom; a second circuit branch including acontrollable diode having a control electrode, and a relay meansserially connected to said controllable diode for energization thereby,said relay means upon energization opening the microwave feedingcircuit; bistable circuit means receiving said control voltage from saidpotentiometer voltage tap for changing output states in responsethereto; and amplifier means coupled between said bistable circuit meansand said control electrode of said controllable diode for triggeringsaidcontrollable diode.

2. A device according to claim 1, further including means effectingimpedance matches between the output circuit of the control voltage andthe input of the bistable circuit means, the output of said bistablecircuit means and the input of said amplifier means and the amplifieroutput and said controlled diode input.

3. A device according to claim 1, characterized in that in parallel withthe controlled diode there is arranged a normally open movable contactwhich is driven to become closed by the energization of said relay.

4. A device according to claim 1, characterized in that between thepotentiometer voltage tap and the bistable circuit, there is inserted acapacitor for blocking the DC component of the control voltage takenfrom the potentiometer.

5. A device according to claim 1, characterized in that said bistablecircuit means comprises a transistor the conductivity of which isdependent on the voltage drawn from the potentiometer.

6. A device according to claim 5, characterized in that said bistablecircuit means further comprises a second transistor which iscommon-emitter connected with respect to the first transistor, the baseof the second transistor being connected to the collector of the firsttransistor through a delay circuit.

7. A device according to claim 6, characterized in that said amplifiermeans comprises a transistor, whose base is connected to the collectorof the first transistor of said bistable circuit means and thecollector-emitter circuit comprises a load resistor across which thecontrol voltage for the controlled diode is drawn.

1. A safety device for sensing and preventing radiation leakage from amicrowave-producing apparatus having a microwave feeding circuit, saiddevice comprising, in combination, a first circuit branch including agas lamp which is sensitive to microwave radiation and which is disposedin a region of possible microwave radiation leakage from themicrowave-producing apparatus, and a potentiometer electrically coupledwith said gas lamp, said potentiometer having a voltage path providing acontrol voltage therefrom; a second circuit branch including acontrollable diode having a control electrode, and a relay meansserially connected to said controllable diode for energization thereby,said relay means upon energization opening the microwave feedingcircuit; bistable circuit means receiving said control voltage from saidpotentiometer voltage tap for changing output states in responsethereto; and amplifier means coupled between said bistable circuit meansand said control electrode of said controllable diode for triggeringsaid controllable diode.
 2. A device according to claim 1, furtherincluding means effecting impedance matches between the output circuitof the control voltage and the input of the bistable circuit means, theoutput of said bistable circuit means and the input of said amplifiermeans and the amplifier output and said controlled diode input.
 3. Adevice according to claim 1, characterized in that in parallel with thecontrolled diode there is arranged a normally open movable contact whichis driven to become closed by the energization of said relay.
 4. Adevice according to claim 1, characterized in that between thepotentiometer voltage tap and the bistable circuit, there is inserted acapacitor for blocking the DC component of the control voltage takenfrom the potentiometer.
 5. A device according to claim 1, characterizedin that said bistable circuit means comprises a transistor theconductivity of which is dependent on the voltage drawn from thepotentiometer.
 6. A device according to claim 5, characterized in thatsaid bistable circuit means further comprises a second transistor whichis common-emitter connected with respect to tHe first transistor, thebase of the second transistor being connected to the collector of thefirst transistor through a delay circuit.
 7. A device according to claim6, characterized in that said amplifier means comprises a transistor,whose base is connected to the collector of the first transistor of saidbistable circuit means and the collector-emitter circuit comprises aload resistor across which the control voltage for the controlled diodeis drawn.